Condensate preheater for waste heat steam generator

ABSTRACT

A condensate preheater for a waste-heat steam generator, including condensate preheater heating surfaces, around which exhaust gas of a gas turbine flows in operation in order to heat up water flowing in the condensate preheater heating surfaces, wherein a respective heat transfer coefficient of the condensate preheater heating surfaces is selected such that a surface temperature of the condensate preheater heating surfaces is above an exhaust gas dew point in operation, wherein the condensate preheater has a condensate preheater inlet and a condensate preheater outlet and the heat transfer coefficient at the condensate preheater inlet is reduced relative to the heat transfer coefficient at the condensate preheater outlet, is provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/EP2014/053078 having a filing date of Feb. 18, 2014 based off of German Application No. DE 102013204396.1 having a filing date of Mar. 13, 2013, the entire contents of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a condensate preheater for the waste heat steam generator of a gas and steam power station, and relates to preventing temperatures from falling below the exhaust gas dew points.

BACKGROUND

The fuels used in gas and steam power stations (GUD) have amongst others variably high sulfur contents. Together with the water proportion which occurs from the combustion of gas or oil, there is a risk that if the temperatures fall below the corresponding exhaust gas dew points, sulfuric acid, sulfurous acids and water will be deposited in the so-called cold part of the waste heat steam generator (in particular in the condensate preheater), and lead to corrosion and finally to component failure.

To prevent temperatures from falling below the exhaust gas dew point and the associated occurrence of water/acid (in particular sulfuric acid) in a gas and steam power station, the condensate temperature must be raised to a corresponding minimum temperature before the condensate enters the condensate preheater of the waste heat steam generator. This is because the heat transmission is determined from the water side, i.e. the pipe wall temperature on the exhaust gas side corresponds approximately to the condensate temperature on the inside. The condensate temperature is in turn set by the defined cooling conditions (cooling type, design of cooling system, ambient conditions etc.). The minimum inlet temperature of the condensate required to prevent it from falling below the exhaust gas dew point has previously mostly been ensured by recirculation of hot water from the condensate preheater outlet to the condensate preheater inlet by means of recirculation pumps or feed water pump take-off

Another possibility is to heat the condensate by an external steam or water-heated preheater before it enters the waste heat boiler. For this, an external hot water or steam source can supply the necessary heat, or the steam heating of the preheater using steam extracted from the steam turbine or hot steam generation takes place in the waste heat steam generator at a temperature level lying above the dew point.

Another possibility, scarcely used, is to make the condensate preheater of (largely) acid-resistant material.

SUMMARY

An aspect relates to a condensate preheater of the type cited initially in which, during operation, the temperature at the condensate preheater heating surfaces does not fall below an exhaust gas dew point.

A further aspect relates to a condensate preheater of this type for a waste heat steam generator comprising condensate preheater heating surfaces around which exhaust gas from a gas turbine flows in operation in order to heat the water flowing in the condensate preheater heating surfaces, a heat transfer coefficient of the condensate preheater heating surfaces is selected such that in operation, a surface temperature of the condensate preheater heating surfaces lies above an exhaust gas dew point. For this, the condensate preheater has a condensate preheater inlet and a condensate preheater outlet, and the heat transfer coefficient at the condensate preheater inlet is reduced relative to the heat transfer coefficient at the condensate preheater outlet.

Previously, in all boiler heating surfaces, the best possible heat transmission was desirable at the condensate preheater, which makes it necessary to increase the condensate inlet temperature accordingly to avoid falling below the dew point, since the heat transmission is determined from the water side.

In contrast, embodiments of the invention is now based on the concept of reducing the heat transmission accordingly, at the individual heat exchanger pipes of the condensate preheater which could be affected if the temperature falls below an exhaust gas dew point due to too low a condensate temperature, so far that the wall temperature of the heat exchanger pipe on the exhaust side rises above the exhaust gas dew point. The reduction in heat transmission is thus applied to condensate preheater heating surfaces which can only reach the minimum permitted pipe outer wall temperature in this way. As soon as the condensate temperature safely reaches the exhaust gas dew point temperature (even under changing operating conditions), this measure may be omitted.

Because the heat transfer coefficient at the condensate preheater inlet is reduced compared with the heat transfer coefficient at the condensate preheater outlet, it can be ensured that only where the temperature would otherwise fall below the dew point is a reduction in the heat transmission achieved, and at the other points the normally desirable maximum heat transmission persists in the interests of keeping the total heating surface of the waste heat steam generator as small as possible.

Suitably, this measure concerns the water vapor dew point which is relevant even in practically sulfur-free fuel gases, and lies in the temperature range of around 50 to 65° C. This depends on the quantity of hydrogen in the fuel gas and increases as the fuel gas and combustion air humidity rises. If the temperature falls below the dew point, the presence of carbon dioxide may lead to corrosion at the heating surfaces at the water vapor dew point.

Above all however, it is useful if the exhaust gas dew point below which the temperature does not fall is a sulfuric acid dew point. The dew point of diluted sulfuric acid, which is higher than the water vapor dew point, lies in the temperature range between 90 and 160° C. and rises with the sulfur content of the fuel.

In an advantageous embodiment, a heat-insulating layer is applied to at least one condensate-carrying part of the condensate preheater heating surfaces lying closest to the condensate preheater inlet. This solution is simple and space-saving.

In a further advantageous embodiment, at least one condensate preheater heating surface lying closest to the condensate preheater inlet is a pipe-in-pipe construction, for example made of C-steel, in which a heat-insulating layer is arranged in an inter-pipe space. Here it is suitable, because it is simple and economic, for the heat-insulating layer to comprise sand, in particular dry sand. The outer pressureless pipe is here designed (as before) with corresponding ribs to enlarge the surface area, the heat-insulating layer and where applicable the diameter of the outer pipe are optimized according to the outer wall temperature of the inner pipe which has already been reached. The inner pipe is designed as before, according to permitted pressure losses, design pressure etc.

The omission of this inlet temperature rise made possible by embodiments of the invention—the reduction in heat transmission due to the additional heat-insulating layer—means that there is no need for condensate recirculation pumps, feed water pump take-offs, external heat exchangers with the associated piping and valves, and hence achieves savings in the associated investment, space requirement, electrical power consumption etc.

Furthermore, embodiments of the invention allow the nominal boiler heater surface in the condensate preheater to be reduced by more than 25% in comparison with a solution of a condensate recirculation, since the reduced condensate mass flow in the condensate preheater over-compensates for the influence of the lower heat transfer coefficient. The amount of tubular steel used would increase slightly on use of the abovementioned exemplary pipe-in-pipe construction, however the outer envelope of the waste heat steam generator would be slightly smaller and no collector would be required. Thus approximately the same investment can be assumed. The benefits in each case of a reduced surface are requirement of the waste heat steam generator and a reduced pump power for the condensate pump (due to lower pressure losses) with a corresponding reduction in the associated energy consumption.

Furthermore, the system availability would increase since active components such as pumps and valves are replaced by a passive heat-insulating layer. The rise in condensate inlet temperature may be omitted as long as it can be guaranteed that the temperature as a whole does not fall below the dew point temperature of the exhaust gas (i.e. the heat quantity available in the exhaust gas is sufficient for condensate preheating as a whole).

Compared with making the condensate preheater from largely acid-resistant material (e.g. stainless steel), the proposed solution is significantly cheaper due to the lower material and production costs.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

FIG. 1 depicts a view of a known condensate preheater with feed water extraction;

FIG. 2 depicts a partial view of a further known condensate preheater with additional condensate recirculation pump;

FIG. 3 depicts a view of an embodiment of a condensate preheater according to the invention and

FIG. 4 depicts a pipe-in-pipe construction for an embodiment of the condensate preheater according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows diagrammatically and as an example a view of a known condensate preheater 12 in the so-called cold part 13 of the waste heat steam generator 2 with feed water extraction. Condensate is supplied via the condensate line 14 to the condensate preheater 12, where it is heated by heat exchange with the exhaust gas 4 flowing through the waste heat steam generator 2. The condensate line 14 branches into lines 15 and 16. Line 15 conducts the condensate to the low pressure drum 17. The condensate diverted into the line 16 is supplied via a feed water pump 18 to the medium-19 and high-pressure drums 20.

FIG. 1 furthermore shows a cold bypass of the condensate preheater via a secondary line 21 and a line 22 for a minimum backflow of the feed water to the condensate line 14, because the feed water pump 18 is in permanent operation and, even if the medium-19 and high-pressure drums 20 are not to be supplied with feed water, a low through-flow of feed water must be guaranteed by the feed water pump 18.

The minimum condensate inlet temperature required to prevent the temperature from falling below the exhaust gas dew point, as shown in FIG. 1, was usually previously ensured by recirculation of feed water from a feed water pump take-off 23. With this system, which comprises comparatively few components, has low complexity and is therefore also cheaper, typically a condensate preheater inlet temperature of around 55° C. can be ensured. This temperature would normally be sufficient to avoid falling below the water vapor dew point.

With the aim of maintaining the temperature above the sulfuric acid dew point, additionally—as shown in FIG. 2—hot water may be returned from the condensate preheater outlet 6 to the condensate preheater inlet 5. However, here in addition to the feed water pump 18 of FIG. 1, a recirculation pump 24 is required.

FIG. 3 shows a diagrammatic view of an embodiment of the condensate preheater 1 according to the present invention. The feed water return and condensate recirculation as shown in FIGS. 1 and 2 may be omitted. Instead, the heat transfer coefficient of the condensate preheater heating surfaces 3 is selected such that in operation, a surface temperature of the condensate preheater heating surfaces 3 lies above an exhaust gas dew point, i.e. above the water vapor dew point or even above the sulfuric acid dew point. This is achieved because the heat transfer coefficient at the condensate preheater inlet 5 is reduced compared with the heat transfer coefficient at the condensate preheater outlet 6, in that a heat-insulating layer 7 is applied to at least one condensate-carrying part of the condensate preheater heating surfaces 3 lying closest to the condensate preheater inlet 5.

FIG. 4 shows a pipe-in-pipe construction 8 according to the invention, as may be used in at least one condensate preheater heating surface 3 lying closest to the condensate preheater inlet 5. In the pipe-in-pipe construction 8, made for example of C-steel, the heat-insulating layer 9, e.g. dry sand 11, is arranged in an inter-pipe space 10. The outer pressureless pipe 25 is, as before, designed with corresponding ribs 26 to enlarge its surface area, the heat-insulating layer 9 and where applicable the diameter of the outer pipe 25 are each optimized according to the outer wall temperature of the inner pipe 27 which has already been reached. The inner pipe 27 is designed as before according to the permitted pressure losses, design pressure etc.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. 

1. A condensate preheater for a waste heat steam generator, comprising: a plurality of condensate preheater heating surfaces around which an exhaust gas from a gas turbine flows during operation to heat a water flowing into the plurality of condensate preheater heating surfaces; wherein a respective heat transfer coefficient of the plurality of condensate preheater heating surfaces is selected such that in operation, a surface temperature of the plurality of condensate preheater heating surfaces lies above an exhaust gas dew point; wherein the condensate preheater has a condensate preheater inlet and a condensate preheater outlet, and the respective heat transfer coefficient at the condensate preheater inlet is reduced compared with the respective heat transfer coefficient at the condensate preheater outlet.
 2. The condensate preheater as claimed in claim 1, wherein the exhaust gas dew point is a water vapor dew point.
 3. The condensate preheater as claimed in claim 1, wherein the exhaust gas dew point is a sulfuric acid dew point.
 4. The condensate preheater as claimed in claim 1, wherein a heat-insulating layer is applied to at least one condensate-carrying part of the plurality of condensate preheater heating surfaces lying closest to the condensate preheater inlet.
 5. The condensate preheater as claimed in claim 1, wherein at least one of the plurality of condensate preheater heating surfaces lying closest to the condensate preheater inlet is a pipe-in-pipe construction in which a heat-insulating layer is arranged in an inter-pipe space.
 6. The condensate preheater as claimed in claim 5, wherein the heat-insulating layer comprises sand. 